If you're a smartphone owner, you're already familiar with the ritual if charging up your device one or more times a day. And the question of how much power you're actually sucking when you do so has probably crossed your mind. It's got to be quite a bit, right?

The report found that the power used to charge a smartphone throughout an entire year is negligible. According to Opower, it takes just 3.5 kilowatt hours per year (an annual energy cost of about $0.41) to charge up an iPhone 5 from 0% to full, year-round. That's pretty cheap.

The rates were pretty close for the Samsung Galaxy S III, which costs roughly $0.53 a year to charge. Opower says the slight discrepancy is probably due to the fact that the Galaxy's battery is larger.

But, though the impact of charging an individual smartphone is pretty small, Opower reminds us that, collectively, they do eat up a lot of power:

"This year alone, Apple expects to sell 170 million iPhone 5s. Those 170 million smartphones will draw enough electricity to power all of the homes in Cedar Rapids, Iowa for a year."

Still, Opower predicts that smartphones may help reduce energy consumption over all, since they're now being used in many cases as substitutes for bulkier, more power-hungry devices, like desktop computers and TVs.

For instance, says Opower, "An iPhone 5 requires 20 times less energy to operate than a typical laptop – and 100 times less energy than a typical 42-inch plasma TV."

“Put simply, says Opower analyst Barry Fischer, "a day spent web-surfing and Facebooking on a smartphone or tablet is a much more energy-efficient day than doing the same on a computer [...] the rise of smartphones, like the iPhone 5, is likely to save energy in American homes as it diverts our time from using far less efficient consumer electronic devices.”

This graphic from Mashable compares the energy used for other consumer electronics relative to the iPhone 5